Biotype Investigation

Late last year, researchers identified a strain of whitefly, Bemisia tabci Q biotype, previously undocumented in the United States. Because Q biotype has been associated with whitefly control problems in Spain, Israel, Morocco and Egypt, many initially feared it would become a significant, uncontrollable pest in the United States. While this prediction is proving to be inaccurate, Q biotype will affect U.S. floriculture nonetheless. GPN’s 12-page special report updates you on what is known so far.
Jim Bethke’s latest research on control methods is summarized in the next three pages. On page 22, Lin Schmale, Society of American Florists, gives an update on how the industry is handling this new pest, and a progress report from the Q Biotype Technical Advisory Committee can be found on page 26.

As of press time, locations in 15 U.S. states (Alabama, Arizona, California, Connecticut, Florida, Georgia, Indiana, Kentucky, Louisiana, Michigan, New Jersey, New York, Oregon, Pennsylvania and Vermont) have tested positive for Q biotype whitefly. Growers are advised to scout crops thoroughly this spring, rotate chemicals properly and seek assistance with resistant whiteflies. Visit www.mrec.ifas.ufl.edu/lso/bemisia/bemisia.htm[1] for more information.

By the time we get this written there may be a sweet potato whitefly biotype for every letter in the alphabet, and like the named tropical storms in the Atlantic, they’ll have to start using the Greek alphabet soon. We’re sure some of you are wondering, besides the regulatory hurdles, what’s the big deal about the Q biotype? We hope to help explain some of the problems we faced and provide at least some solutions. We cannot possibly cover everything, and we promise that there is much more to come. Be sure to check out the articles from Lin Schmale, Society of American Florists (page 22) and Lance Osborne, University of Florida (page 26) to get more of your questions answered.

We have just finished a round of efficacy trials with various chemicals here at University of California Riverside (UCR), and we’d like to provide some hope. It is clear that the Q biotype is present in the United States, and ornamental growers have it. Part of the good news is at this point growers seem able to control it. There is some evidence Safari (dinotefuran, Valent U.S.A. Corporation) is active against this pest, so the California Department of Food and Agriculture used it in an eradication effort at infested sites in California. In addition, Q biotype has been in Europe for some time, and products such as spiromesifen, U.S. trade name Judo (OHP, Inc.), are controlling the pest there. Our first trials also showed efficacy with several common insecticides used against Q biotype.

What Is Biotype?

A biotype is a group of organisms that have the same genetic constitution. They are more closely related to each other in their genetic makeup than to other populations of the same species. Many times a biotype is a regionally isolated or separated population that has been interbreeding for a long time and may have become genetically different than populations of the same insect in another region. At times, they may be different enough to be separated into a completely new species, as the B biotype was into the silverleaf whitefly, Bemisia argentifolii.

Biotypes may be genetically different enough to have very different detoxification mechanisms to insecticides. For example, laboratory leaf dip bioassays have demonstrated that Q is resistant to the IGRs Distance (pyriproxifen, Valent U.S.A. Corporation) and Talus (buprofizen, SePRO Corporation) and tolerant to several of the neonicotinoids including Flagship (thiamethoxam, Syngenta Professional Products), Marathon (imidacloprid, OHP, Inc.) and Tristar (acetamiprid, Cleary Chemical Corporation). In addition to the differences in resistance mechanisms, it is well known that the B biotype causes silvering of squash leaves. Q, however, does not cause silvering. What’s worse is that Q is a very efficient vector of serious viruses in Europe such as the tomato yellow leaf curl virus.

Relative Importance

You may ask what importance all these biotypes have to you. The full implications are not yet known, especially with the Q biotype. Let me remind you, however, that the A biotype was common in the United States and was not really a significant problem for most growers, just an occasional nuisance. On ornamentals, it was common to find the greenhouse whitefly, Trialeurodes vaporariorum, and control efforts against this whitefly were common. However, on ornamentals here in California in the late 1980s we observed a significant shift from the greenhouse whitefly to the silverleaf whitefly or the B biotype. In addition, the same was occurring in cotton and vegetables where the B biotype was replacing the A biotype. Soon afterward, the B biotype became a very serious pest for the ornamental, cotton and vegetable industries. Therefore, we must assume that the Q biotype is potentially a serious threat for the floriculture industry, and the cotton and vegetable industries are certainly taking it very seriously.

Efficacy Trials

We’ve identified and colonized several Q populations on two different host plants from four counties in California. None of the populations in commercial situations are exclusively Q. They always have some proportion of the B biotype present. We have selected a population that is high in Q (97 percent) with which to conduct our greenhouse and field efficacy trials. In addition, Tim Dennehy, University of Arizona, has provided a pure Q colony to the quarantine facility at UCR. Dennehy has selected for the Q strain by treating his colony with Distance. Research has shown that under pesticide-selection pressure the proportion of Q increases, and without pesticide-selection pressure, the population moves toward the B biotype.

We evaluated the efficacy of several insecticides against young nymphs and adults of Q in four trials. In trials against whiteflies we use a clip-cage to expose 10-20 adult whiteflies for up to 48 hours to the underside of the leaf surface of a host plant, usually poinsettia. The adult females lay a batch of eggs on the leaf. In about nine days they hatch, and we count the number of early instars as a pretreatment sample. Following the pre-treatment count, we make our treatment applications. Efficacy is determined at adult emergence, i.e., adults emerge at high rates on untreated plants, and few, if any, emerge from plants treated with effective insecticides.
In one of the trials, we also confined adults to treated plants and assessed adult mortality. Using these methods, we have determined products we are confident will kill the whiteflies. It is up to the grower then to make efficient applications for good efficacy.

Results

Several products worked very well, as illustrated in Figure 1, left. Avid/Talstar (Syngenta Professional Products/FMC Corporation), Avid, Sanmite (The Scotts Company LLC), Distance and Judo are not significantly different from one another and provided the best results. Indeed, Judo and the tank mix of Avid plus Talstar provided 100-percent mortality. When compared to the control, Talus and Talstar were unable to control Q. Although DuraGuard (Whitmire Micro-Gen) and Tame (Valent U.S.A. Corporation) were significantly different than the control, they did not provide appreciable control because more than 20 percent of the whitefly nymphs were able to complete development.

One caveat to this data: We have a trial in progress that confirms Distance will kill B biotype whiteflies very well but is less effective against Q than in the earlier trial. The trial in progress shows that Q is quite resistant to Distance, as has been observed in other studies in the United States and Europe.

Figure 2, above, illustrates that the neonicotinoids are effective against Q as both sprays and drenches. All treatments are significantly different from the control, but some of the products allow a significant number of nymphs to complete development to the adult stage. Best results to date have been with Safari as a drench or spray and Tristar as a spray. All other products, whether applied as a spray or drench, allowed 20 percent or more of the nymphs to complete development. Drenches of Celero (Arysta LifeScience) and Marathon were inadvertently made at lower-than-labeled rates and Á probably would have performed better at the higher rate. Trials against Q are in progress at a commercial greenhouse to confirm efficacy of the neonicotinoids as drenches at recommended rates on poinsettias.
We also tried the neonicotinoids as a drench against the pure Q colony (see Figure 3, page 19). All four products caused a significant reduction in the number of emerging adults from treated nymphs, with Safari causing 100-percent mortality. On these same plants we clip-caged adults for 48 hours to determine the neonicotinoids’ effects on adult whiteflies. The adults were clip-caged on the plants 12 days after the pesticide treatment to allow successful translocation of the products. Safari and Flagship caused 100-percent mortality (see Figure 4, page 19).

Recommendations

It is important for you to be aware of whitefly populations within your facility and be ready to modify the management of these pests should you encounter problems. You may be seeing the development of resistance in B biotype, or you may be seeing more Q than in previous years. In any event, you need to adjust your spray and monitoring programs wisely. As always, we recommend reviewing all aspects of your IPM program. Use the proper monitoring techniques — yellow sticky cards, weeding, spot treating, etc. — and rotate modes of action.

If you have a successful program that controls whiteflies, I suggest not changing it unless you encounter problems. However, if you are having problems, it is probably time to determine which biotype you are dealing with. Keep in mind that growers in Europe and infested areas of the United States are able to manage this pest using a combination of tactics and an emphasis on resistance management. Therefore, you may wish to substitute some of the compounds effective against Q.

In our trials, foliar applications of Avid, Avid/Talstar, Tristar, Safari, Sanmite, Distance and Judo, and drench applications of Safari and Flagship were effective against early instars of Q on poinsettias. Safari and Flagship drenches were effective against adults. Although results herein suggest that Distance is effective, caution must be taken because research has shown that the Q biotype is highly resistant to its mode of action.
New trials are in progress to investigate the effective use of organophosphates and pyrethroids alone and in combination and neonicotinoid drenches before and during color in poinsettias. Other researchers are also embarking on efficacy trials against Q as well. We should have a significant amount of information for the industry forthcoming.

Jim Bethke is staff research associate, Frank Byrne is assistant researcher and Richard Redak is professor and entomologist in the Department of Entomology, University of California, Riverside, Calif. Bethke can be reached by phone at (951) 827-4733 or E-mail at bethke@ucr.edu[5].